Conventionally, there has been disclosed a molten metal holding furnace for holding a molten metal of aluminum in patent document 1. The molten metal holding furnace disclosed in Patent Document 1 has a furnace body for holding a molten metal. The furnace body has a side wall formed with a hole through which a heating tube is inserted into the molten metal.
Another heating tube which is available for the molten metal holding furnace is disclosed in patent document 2.